Between ages 18 and 24, the prevalence of criminal offending decreases sharply. It is commonly believed that delinquents become desisters because of the developmental transitions of young adulthood: marriage, a new family, and employment. However, there are few empirical descriptions of this process. At the same time that the majority of delinquents abandon crime for more pro-social lifestyles, a smaller number of individuals continue their crime careers into young adulthood and beyond. These persisters are thought to have high offense rates and to be often violent. It is not known why criminal persisters are exempt from the taming influence of adult transitions that presumably help others to desist. Criminological theories, preoccupied with the origins of antisocial behavior, fail to account for the marked divergence in the lifecourses of delinquents during young adulthood, one of the most robust facts known about crime. The proposed program of research offers innovations in theory and empirical research. We propose to follow up at age 21 the 1,000 members of the Dunedin New Zealand Multidisciplinary Health and Developmental Longitudinal Study, and their new spouses and children. The sample is a representative birth cohort of males and females whose antisocial behavior has been previously studied at ages 3, 5, 7, 9, 11, 13, 15, and 18. To complement the existing data base of social, psychological, and biological variables on this cohort, we shall collect new data on: self-reported, peer-reported and official crime, mental disorder, substance use, sexual & reproductive behavior, and intra-familial antisocial behavior toward spouses and children. These outcomes will be related to new data on the timing and quality of young adult transitions: education, work, finances, cohabitation and marriage, child-bearing, parenting, social networks, residence change, and illness and disability. The research will be the first to describe the natural history of antisocial behavior in the same individuals, male and female, from infancy to adulthood. The collection and analysis of data in this program of research are guided by a new theory developed during the first 6 years of NIMH support. The theory is both developmental and taxonomic in nature. It proposes two distinct types of antisocial involvement: "Life-Course Persistent Antisocial Behavior" and "Adolescence-Limited Delinquency". I have argued that the two types differ in etiology, developmental course, demographic distribution, prognosis for intervention, and in the classification of their behavior as either pathological or adaptive. The follow up of the NZ sample at age 21 will allow tests of a set of point predictions generated from the theory. By linking criminal desistence and criminal persistence to life history data from birth to young adulthood, the proposed research aspires to make some advances in research design and theory-testing, and to yield findings that are of practical relevance for intervention policy.